This invention relates generally to free-electron lasers and, more particularly, to techniques for increasing the efficiency of free-electron lasers. By way of background, a free-electron laser generates coherent light when bunches of free electrons, accelerated to near relativistic velocities, i.e. near light speed, are passed through an alternating magnetic field known as a wiggler. Considerable energy is required to accelerate the electrons to near light speed, and the overall efficiency of such a system, referred to as the wallplug efficiency, is typically only a few percent.
Basically, a free-electron laser has two principal components: the laser itself, in which some of the energy of the electrons is converted into light energy, and an electron acceleration-recirculation system. Unfortunately, these two components have conflicting requirements that give rise to inefficiencies in the overall system. The acceleration-recirculation system ideally requires that each bunch of accelerated electrons be spread over a relatively long time interval but over a relatively small energy range. This makes for efficiency of energy recovery from electrons emerging from the laser, and avoids electron beam instabilities. On the other hand, the free-electron laser ideally needs a large peak current to extract energy efficiently from the electron beam. A large peak current can only be achieved if the electrons are bunched together in time, rather than spread out as is desirable for the acceleration-recirculation system.
Another incompatibility arises because the free-electron laser not only reduces the average energy of the electron beam, but also spreads each bunch of electrons across a wider energy range. This renders each bunch of electrons less suitable for reintroduction to the acceleration-recirculation system. Typically a free-electron laser introduces an energy spread equal to about twice its extraction efficiency, and the wider spread is difficult to accommodate in the acceleration-recirculation system.
It will be appreciated from the foregoing that there is a need in the field of free-electron lasers for an overall system with a greatly improved wallplug efficiency. The present invention achieves this goal by meeting the separate efficiency requirements of the free-electron laser and the acceleration-recirculation system.